The disclosure generally relates to low-pressure mercury discharge lamps. More specifically, the disclosure relates such lamps having a lamp fill including mercury, bismuth and indium, and methods of dosing the lamp with the fill material using substantially solid mercury-containing pellets of high purity, uniform size, and uniform composition.
Fluorescent lamps are well known and contain a vaporizable lamp fill including mercury. In the manufacture of such lamps, it is necessary to introduce very small amounts of mercury into the light emitting chamber of the lamp. For example, some fluorescent lamps include only about 0.1 mg up to about 10 mg of mercury, depending on the size of the lamp. While it is possible to introduce liquid mercury directly into the lamp, it is very difficult to obtain precise doses of such small quantities of mercury due to the high surface tension of mercury. Consequently, lamps dosed by using liquid mercury usually contain more mercury than is needed for operation of the lamp leading to environmental concerns in the disposal of the lamps. To address these concerns, mercury has been combined with other elements to form a substantially solid lamp fill material, thereby easing the handling and dispensing of the material while providing a means for dosing precise amounts of mercury into the lamp.
Another concern is maintaining the mercury vapor pressure at a level such that the lamp operates efficiently within a range of temperatures. The mercury vapor atoms convert electrical energy into ultraviolet radiation. The mercury vapor pressure is preferably in the range of approximately 2×10−3 to 2×10−2 Torr and optimally, about 6×10−3 Torr. The ultraviolet radiation is in turn absorbed by a phosphor coating on the interior of the lamp wall and converted to visible light. As the operating temperature of the lamp increases, the mercury vapor pressure increases and more of the ultraviolet radiation is self-absorbed by the mercury, thereby lowering the efficiency of the lamp and reducing light output. Thus, the mercury vapor pressure must be controlled. Conventionally, in one type of fluorescent lamp the mercury vapor pressure is controlled by controlling the temperature of the lamp. In another type of fluorescent lamp, the mercury vapor pressure is controlled by adding a mercury vapor pressure regulating material to the lamp.
Lamps in which a mercury vapor pressure regulating material is utilized for mercury vapor pressure control typically operate with a cold spot temperature of above 75° C. and generally have a small diameter. Such lamps are known as “compact lamps”, and typically require an amalgamative metal in addition to mercury in the lamp fill for mercury vapor pressure control. U.S. Pat. No. 4,157,485 discloses an indium-bismuth-mercury amalgam that is used to control the mercury vapor pressure in a low pressure mercury vapor discharge lamp, i.e., fluorescent lamp, over a wide temperature range. The goal of the amalgam is to maintain the mercury vapor pressure at 6×10−3 Torr (the optimum vapor pressure for a fluorescent lamp) over as wide of temperature range as possible. Although the indium-bismuth amalgam maintains a lower mercury vapor pressure at room temperature than pure mercury, the mercury vapor pressure is sufficient for the lamp to start. At temperatures above about 40° C. (which is the optimum mercury vapor pressure for a lamp with pure mercury) the efficiency of a lamp containing only mercury decreases while a lamp containing an indium-bismuth amalgam remains greater than 90% of the possible light output for temperatures up to about 130° C. The upper temperature limit is determined primarily by the chemical composition and the mercury content of the amalgam. U.S. Pat. No. 4,157,485 discloses an indium-bismuth amalgam wherein the ratio of atoms of bismuth to atoms of indium is between 0.4:0.6 and 0.7:0.3 and the ratio of atoms of mercury to the sum of the atoms of bismuth and indium is between 0.01:0.99 and 0.15:0.85.
The composition of the indium-bismuth-mercury pellets in commercial typically use is 28 to 32 weight percent indium, 64 to 69 weight percent bismuth and 1.5 to 5.0 weight percent mercury. However, the manufacture and production of lamps using an amalgam with this composition is difficult because of a small amount of liquid amalgam present in the pellet. The pellets agglomerate at substantially room temperature and are difficult to separate. Thus the pellets are not “free flowing”, i.e., the pellets tend to stick together when in contact and will not roll over other pellets. The self-agglomeration may occur immediately after the pellets are manufactured or it may occur after several weeks have passed. The poor flow properties of the abovementioned amalgam composition cause significant problems with handling, dosing and lamp manufacture. Self-agglomeration of these amalgams can cause waste in the lamp manufacturing environment and limit the use of these amalgams.
Accordingly, it is an object of the disclosure to address the above-mentioned problems and to provide novel lamp fill materials, methods of dosing fluorescent lamps, and methods of improving the handling characteristics of lamp fill materials containing mercury. It is a further object to provide novel lamp fill materials forming free flowing solids. It is yet another objection of the present disclosure to provide pellets having a composition of mercury, bismuth, indium and another metal wherein the pellets are free flowing an include material that regulates the mercury vapor pressure during operation of fluorescent lamps. It is another object of the disclosure to regulate the mercury vapor pressure within a low pressure mercury discharge lamp with indium-bismuth-mercury amalgam. It is still a further object of the disclosure to improve the manufacture of low pressure mercury vapor discharge lamps with an indium-bismuth-zinc-mercury amalgam. It is yet a further object of the disclosure to provide a novel method of introducing a precise amount of mercury into an amalgam-controlled fluorescent lamp.
These and many other objects and advantages of the disclosure will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following description.